There are no large caverns inside Comet 67P/Churyumov-Gerasimenko.
ESA's Rosetta mission has made measurements that clearly demonstrate
this, solving a long-standing mystery.

Comets are the icy remnants left over from the formation of the
planets 4.6 billion years ago. A total of eight comets have now
been visited by spacecraft and, thanks to these missions, we have
built up a picture of the basic properties of these cosmic time
capsules. While some questions have been answered, others have
been raised.

Comets are known to be a mixture of dust and ice, and if fully
compact, they would be heavier than water. However, previous
measurements have shown that some of them have extremely low
densities, much lower than that of water ice. The low density
implies that comets must be highly porous.

But is the porosity because of huge empty caves in the comet's
interior or it is a more homogeneous low-density structure?

In a new study, published in this week's issue of the journal
Nature, a team led by Martin P??tzold, from Rheinische Institut
f??r Umweltforschung an der Universit??t zu K??ln, Germany, have
shown that Comet 67P/Churyumov-Gerasimenko is also a low-density
object, but they have also been able to rule out a cavernous interior.

This result is consistent with earlier results from Rosetta's
CONSERT radar experiment showing that the double-lobed comet's
'head' is fairly homogenous on spatial scales of a few tens of metres.

The most reasonable explanation then is that the comet's porosity
must be an intrinsic property of dust particles mixed with the ice
that make up the interior. In fact, earlier spacecraft
measurements had shown that comet dust is typically not a
compacted solid, but rather a 'fluffy' aggregate, giving the dust
particles high porosity and low density, and Rosetta's COSIMA and
GIADA instruments have shown that the same kinds of dust grains
are also found at 67P/Churyumov-Gerasimenko
<http://blogs.esa.int/rosetta/2015/04/09/giada-investigates-comets-fluffy-dust-grains/>.

P??tzold's team made their discovery by using the Radio Science
Experiment (RSI) to study the way the Rosetta orbiter is pulled by
the gravity of the comet, which is generated by its mass.

The effect of the gravity on the movement of Rosetta is measured
by changes in the frequency of the spacecraft's signals when they
are received at Earth. It is a manifestation of the Doppler
effect, produced whenever there is movement between a source and
an observer, and is the same effect that causes emergency vehicle
sirens to change pitch as they pass by.

In this case, Rosetta was being pulled by the gravity of the
comet, which changed the frequency of the radio link to Earth.
ESA's 35-metre antenna at the New Norcia ground station in
Australia is used to communicate with Rosetta during routine
operations. The variations in the signals it received were
analysed to give a picture of the gravity field across the comet.
Large internal caverns would have been noticeable by a tell-tale
drop in acceleration.

ESA's Rosetta mission is the first to perform this difficult
measurement for a comet.

"Newton's law of gravity tells us that the Rosetta spacecraft is
basically pulled by everything," says Martin P??tzold, the
principal investigator of the RSI experiment.

"In practical terms, this means that we had to remove the
influence of the Sun, all the planets ??? from giant Jupiter to the
dwarf planets ??? as well as large asteroids in the inner asteroid
belt, on Rosetta's motion, to leave just the influence of the
comet. Thankfully, these effects are well understood and this is a
standard procedure nowadays for spacecraft operations."

Next, the pressure of the solar radiation and the comet's escaping
gas tail has to be subtracted. Both of these 'blow' the spacecraft
off course. In this case, Rosetta's ROSINA instrument is extremely
helpful as it measures the gas that is streaming past the
spacecraft. This allowed P??tzold and his colleagues to calculate
and remove those effects too.

Whatever motion is left is due to the comet's mass. For Comet
67P/Churyumov-Gerasimenko, this gives a mass slightly less than 10
billion tonnes. Images from the OSIRIS camera have been used to
develop models of the comet's shape and these give the volume as
around 18.7 km^3 , meaning that the density is 533 kg/m^3 .

Extracting the details of the interior was only possible through a
piece of cosmic good luck.

Unfortunately, prior to 2014, the RSI team predicted that they
needed to get closer than 10 km to measure the internal
distribution of the comet. This was based on ground-based
observations that suggested the comet was round in shape. At 10 km
and above, only the total mass would be measurable.

Then the comet's strange shape was revealed as Rosetta drew
nearer. Luckily for RSI, the double-lobed structure meant that the
differences in the gravity field would be much more pronounced,
and therefore easier to measure from far away.

"We were already seeing variations in the gravity field from 30
km away," says P??tzold.

In September, Rosetta will be guided to a controlled impact on the
surface of the comet. The manoeuvre will provide a unique
challenge for the flight dynamics specialists at ESA's European
Space Operations Centre (ESOC) in Darmstadt, Germany. As Rosetta
gets nearer and nearer the complex gravity field of the comet will
make navigating harder and harder. But for RSI, its measurements
will increase in precision. This could allow the team to check for
caverns just a few hundred metres across.

Notes for Editors

"A homogeneous nucleus for comet 67P/Churyumov???Gerasimenko from
its gravity field <http://dx.doi.org/10.1038/nature16535>,/" by M.
P??tzold et al. is published in the journal Nature,
doi:10.1038/nature16535